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(2017 Aug. 12)
(2017 Aug. 21)
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[2016 Mar. 9]
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[2010 March 1021]
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SOLAR ECLIPSE BASICS"To witness a total eclipse of the Sun is a privilege that comes to but few people"
(Isabel M. Lewis, 1924)
« Also read "Why See a Total Eclipse of the Sun?" »
Solar Eclipses: Partial or complete obscuration of Sun by Moon
(Left: Uneclipsed; Left-Center: Partial; Right-Center: Annular; Right: Total)
Other Pages with Related Solar Eclipse Material
Eclipse Phenomena • Eye Safety • Sun's Image Size • Exposure Guide • Times for No Trailing • Glossary
Coincidence of Size The Sun is about 400 times larger than the Moon but also 400 times further from Earth than the Moon. (The Sun's and Moon's diameters are about 1,391,000 and 3,476 km respectively or about 400:1; the Sun's and Moon's distances from Earth are about 149,600,000 and 384,500 km respectively or about 389:1.)
This causes the Sun to appear nearly the same size as the Moon as seen from Earth (an angle of one-half degree), an amazing coincidence. See Fig. 1.
Therefore, if the Moon should pass in front of the Sun during its monthly motion on the sky, it is possible for the Moon to partially or completely block the Sun as seen from Earth. This causes an eclipse of the Sun. These eclipses, therefore, can either be partial or central respectively.
On the average, the Sun appears to be a few per cent larger than the Moon so that the Moon would normally block the entire solar disk if the eclipse was central. However, slight variations in both the Sun's and Moon's distances from Earth can cause the Moon to appear either slightly smaller or slightly larger than the Sun.
Hence, aside from partial eclipses, there are two basic types of central eclipses of the Sun, annular and total.
Annular Eclipses If the Moon appears too small to cover the Sun, an annular eclipse occurs because an annulus (or ring) remains around the eclipsed Sun (see Fig. 2).
This bright ring of sunlight surrounding the Moon's disk does not permit many of the phenomena associated with total eclipses to appear (such as the Sun's chromosphere and corona). The bright annulus is also dangerous to look at so observers must use a safe solar filter during all parts of an annular eclipse. This includes the uneclipsed Sun (both before and after the eclipse), the Sun's partially eclipsed disk, and the annular phase at mid-eclipse.
Total Eclipses If the Moon appears large enough to cover the Sun, a total eclipse occursthe Sun's disk is completely hidden by the Moon if the observer is in the umbral shadow (see Fig. 3).
During this time observers can see solar prominences, the faint inner solar atmosphere (chromosphere), and the Sun's glorious outer atmosphere (coronal halo), which are too faint to be seen when any part of the bright solar disk is exposed. In addition, the sky usually darkens enough to reveal bright stars and planets while an eerie twilight glow circles around the horizon.
The only time it is safe to look in the direction of the Sun without a safe solar filter is during totality. (In fact, if you use a solar filter during totality, you will not see anything!)
The umbra is only about a hundred or so miles wide at the Earth's surface but the penumbral shadow is several thousand miles wide. The observer must be completely within the umbral shadow to see a total eclipse. Otherwise, the observer will only see a partial eclipse (safe solar filter required) and all the incredible sights of the total eclipse will not be seen!
Duration of Totality Because the umbra races across the Earth's surface so quickly (usually at over 1,000 miles per hour!), totality typically lasts only a few minutes. About 7½ minutes is the greatest possible duration and that happens infrequently. (Maximum duration of an annular eclipse is about 12 minutes.) The last seven minute eclipse was in 1973, the next will not occur until 2150! (See Table 1.)
Eclipse Paths caused by the Moon's shadow sweeping across the Earth are very narrow but very long. The animation in Fig. 4 shows an example of the path of the Moon's umbral and penumbral shadows during a total solar eclipse. The eclipse is that of 2017 August 21, the next total eclipse that will occur in the United States. (See Prolonged Eclipse Drought for North America below.) Upper right corner shows the Universal Time (Greenwich Civil Time) as the animation runs. Lower right corner shows instantaneous duration of the total eclipse.
The penumbra appears as a large grayish region thousands of kilometers or miles in diameter sweeping across the Earth from west to east. Everyone within the penumbra's path sees a partial eclipse of the Sun. Outside the penumbral path, no eclipse is visible.
The Moon's umbral shadow appears as a very small black dot (typically only a few hundred kilometers or miles wide) at the center of the penumbra. The dot moves quickly so look carefully since the Moon's shadow moves rapidly across the Earth's surface at velocities of several thousand kilometers or miles per hour. Only those within the narrow umbral path see a total eclipse, which usually lasts for only a few minutes at a given location.
The moving dark blue area shows the nighttime areas of the Earth. From start to finish, the penumbra can take more than five hours to sweep across the Earth.
Short Movie of Lunar Shadow View a short movie of the Moon's shadow sweeping across lower Baja, Central American and Northern Venezuela during the 1991 July 11 total solar eclipse! We produced the movie from GOES-7 Satellite images. (See National Optical Astronomy Observatories.)
Frequency of Eclipses As the Moon orbits the Earth, the Sun, Earth and Moon approximately line up about every 29.5 days. At this time the Moon is either at New Phase (when the Moon is between the Earth and Sun) or Full Phase (when the Earth is between the Moon and Sun). However, the Moon usually does not pass in front of the Sun (causing a solar eclipse), nor does the Moon pass into the Earth's shadow (causing a lunar eclipse) since the Moon's orbit is inclined about 5 degree to the plane of the Earth's orbit around the Sun.
Solar and lunar eclipses can only occur when the Moon is near (±18°) one of the two points where the Moon's and Earth's orbits cross (called nodes). This allows the Moon to pass at least partially in front of the Sun (a solar eclipse), or at least partially move into the Earth's shadow (a lunar eclipse). That is, for eclipses to occur the Moon must be at New or Full Phase and the line of nodes must point toward Sun (points 1 or 3 in Fig. 5). Otherwise the Moon will not lie in the plane of the Earth's orbit and eclipses will not be possible (such as point 2 in Fig. 5).
Every six months these nodes line up with the Sun's direction and solar eclipses become possible at the time of New Moon. Likewise lunar eclipses become possible at the time of Full Moon. These two intervals of time when solar and lunar eclipses are possible are called eclipse seasons. Because the Moon's orbit slowly rotates in space, the nodes regress slowly with a period of approximately 18.6 years causing the two eclipse seasons to occur about 19 days earlier each year.
For example,during 20122013 the eclipse seasons occur in May and November.
The Number of Eclipses Per Calendar Year Thus, each calendar year usually has four eclipses (the minimum number), two of the Sun and two of the Moon (as in both 2004 and 2005). These eclipses occur as solar-lunar eclipse pairs, each eclipse pair separated by about six months. However, sometimes there may be as many as seven solar or lunar eclipses of some type (as in 1982). This can happen if the Moon can move from New back to New (or Full back to Full) before the Earth-Moon direction gets too out-of-line with the Sun to prohibit an eclipse. Examples of eclipse frequencies are in Table 2 for the years 2000 to 2012.
Note: The number of lunar eclipses per calendar year can vary from two to five if penumbral lunar eclipses are counted. (In a penumbral lunar eclipse, the Moon only passes through the lighter, penumbra or outer part of the Earth's shadow.) Since penumbral eclipses are often inconspicuous, the number of conspicuous (i.e., umbral) lunar eclipses can range from zero to three each calendar year. The number of solar eclipses possible each calendar year of all types is also two to five. Counting only annular and total solar eclipses, the number is again none to three as with umbral lunar eclipses.
The Saros Eclipses occur in families. The Saros cycle is a period of about 6,585.3 days (18 years 11 days 8 hours). Two eclipses separated by one Saros cycle have similar geometry (similar duration, same time of year, etc.).
The periodicity and recurrences of solar eclipses as governed by the Saros is useful for organizing eclipses into families. A typical Saros series lasts about 12 to 13 centuries and contains 70 or more eclipses.
Prospects of Seeing Eclipses Solar eclipses are thus more numerous than observable lunar eclipse. But more people see lunar eclipses than annual or total solar eclipses! Why? Since penumbral lunar eclipses are inconspicuous, the number of observable solar eclipses (partial, annual or total) over a period of time typically outnumbers observable umbral lunar eclipses (partial or total) by about three to two. However, lunar eclipses are more common for any one location on Earth as long as the Moon is above your horizon at the time of the eclipse. That is, about one half of the Earth sees the lunar eclipse.
To see an annular or total solar eclipse, the Moon's shadow must pass over your location. Thus, even partial eclipses are visible only over a limited area. And to see an annular and especially total eclipse, the eclipse path is even more limitedoften less than 250 km (150 miles) wide.
Although total solar eclipse occur about every 1½ years, a given place on Earth only averages one total eclipse of the Sun every 350 years. (Sometime two total solar eclipses may occur for the same spot within a short interval of time as will occur for Tallahassee, Florida in 2045 and 2052!)
Prolonged Eclipse Drought for North America! The last total eclipse visible in the continental United States occurred in the Northwestern states in 1979, the last in the USA in 1991 (Hawaii), and the last in the Western Hemisphere in 1998 (northern South American coast and the Caribbean). The next total solar eclipse for the North America continent is in 2008 but it crosses northern Canada and northern Greenland so will be not be easily seen by most people! So, the next "accessible" total solar eclipse for North America is not until 2017. This eclipse will be a "big one" for the USA since it crosses the entire continent from west to east. (See animation in Fig. 4.)
Read article about the big eclipse drought in the USA (pdf document).
For lots more eclipse information, see Fred Espenak's Eclipse Home Page.
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